Existing quadrature couplers for matching and combining of monolithic microwave integrated circuit (MMIC) power amplifiers are limited in bandwidth to about 3:1, due to fundamental limits, and are furthermore limited in size by the necessity of the structure being approximately a quarter wavelength at the center frequency of operation. There is therefore a need for ultra-small and wider bandwidth directional couplers useful in a number of applications. For instance, quadrature couplers are used in mixers and matching other nonlinear components such as limiters. Thus, microwave directional couplers are important and versatile components used in a large variety of applications, including mixers, power splitters and combiners, test equipment, and many others.
Directional couplers are four-port circuits which, in the simplest instance, comprise a pair of coupled lines with an electromagnetic coupling between the lines. The wave propagation down these lines can be described in terms of two modes: an even mode and an odd mode. These waves may also propagate down the lines with different velocities. These types of couplers may include edge coupled, multiple edge coupled, broadside coupled, and spiral edge coupled circuits. In general, tighter coupling is needed for broader bandwidth operation. As will be appreciated, coupling strength is determined in part by how close together the lines are situated. However, there is a practical limitation on how close together the lines can be made.
One common arrangement for coupled lines has long rectangular strips of metal placed side by side on a flat or planar dielectric material in the so-called edge coupled configuration. The coupling strength in this case may be severely limited, but can be increased by using multiple, appropriately interconnected, pairs of these lines. One such arrangement is called the Lange coupler. Arranging the conducting strips so that one is stacked on top of the other, in a so-called ‘broad side coupled configuration’, can further increase the available coupling. To be of practical use, the dimensions and configuration of the two lines must be such that the coupling is the desired strength, and that the structure is well matched to each of the four connecting ports, a typical value being 50 ohms. Simultaneously fulfilling these criteria may not be possible with achievable dimensions.
With current planar technology, it is a relatively straightforward task to achieve a matched impedance for the four ports in a single section style coupler. However, a single section coupler, theoretically, has a band limit, which is typically a tradeoff with the amount of allowed over-coupling. For instance, a peak-to-peak over coupling of 1 dB has a maximum theoretical bandwidth of about 2.4:1. The Lange coupler utilizes an edge coupling technique and seeks to increase the bandwidth by tightening the coupling utilizing interdigitated interconnections between planar transmission lines. However, this technique can only be extended so far. A single Lange coupler section is limited to <2 dB of over coupling. The edge coupled Lange coupler is also relatively large due to the requirement to be approximately one quarter wavelength long at the center frequency of operation.
By using multiple quarter-wave coupled line sections it is possible to greatly increase the bandwidth of operation. However, this further complicates miniaturization for integrated circuit applications. For instance, while single section quadrature couplers can only theoretically have a maximum bandwidth of 2.4:1, one can increase the bandwidth of the directional coupler by adding sections. Typically in a multi-section coupler, there are at least three sections, with the center section requiring the most tightly coupled lines. When trying to increase the bandwidth of the aforementioned Lange coupler, the size of the center section is approximately as large as the outer two sections, which dramatically increases the size of the directional coupler.
Therefore, there is a need in a multi-section coupler to minimize the size of the center section while at the same time providing it with a tight coupling characteristic.